1. Field of the Invention:
The present invention relates generally to industrial process control, and more specifically to a technique for controlling a cure process for fiber composite materials.
2. Description of the Prior Art:
In the aerospace industry, as well as many other industries, fiber composite materials are continuing to become of increasing importance in the fabrication of a wide variety of products. Use of composite materials often provides higher strength to weight ratios than previously obtainable. In addition, complex shapes, which cannot easily be formed in other materials, are achievable.
Forming composites requires that thermosetting polymer resins (matrices) be cured (polymerized) in situ with reinforcing fibers. The challenge for a cure control system is to yield an acceptably cured part while controlling only temperature, pressure and vacuum over time. The goals of a successful cure are good consolidation with low porosity (voids) and high degree of cure. To do so successfully requires managing the interactions of temperature distribution, degree of cure, laminate thickness, and void content in real time.
The typical approach to developing a cure process uses trial and error with past experience to arrive at a successful cure process, or recipe. Thicker parts are particularly difficult, since a slow or controlled heat up rate is required to cure the part, but it is difficult to predict what heat rate to use that will efficiently cure the part while not overdriving the heating and causing uncontrollable exotherm.
Once a successful process recipe has been found, industrial PID (proportional-integral-derivative) feedback controllers are typically used to follow the specific time-temperature-pressure-vacuum sequence over time. While conventional controllers do an admirable job of carrying out the recipe, they are programmed to faithfully follow the fixed and predetermined sequence of set points throughout the cure. Thus, they are not flexible or adaptive to inevitable variations in materials batch and condition that occur within a manufacturing setting. Other than whatever time that is necessary to heat up to a soak or dwell and cool down the part, the process is followed rigidly and does not vary.
More sophisticated approaches to composite cure control are described in U.S. Pat. Nos. 5,345,397 and 5,453,226. The processes described in these patents utilize computer control to modify operation of the cure process based upon workpiece parameters measured during the process. This can provide improved process control. However, techniques such as these suffer from a number of important drawbacks.
The ""397 patent utilizes a type of modeling to predict or estimate laminate temperatures in real time. This is done to calculate an autoclave set point. However, this patent appears to be directed only to relatively thin parts, and does not perform any look ahead to compensate for the exotherm issue. In addition, the description of the ""397 patent does not apply any techniques to a press, nor control pressure or otherwise react to viscosity of the workpiece.
The ""226 patent selects from a set of preselected cure cycle recipes. If temperature travels outside a selected range, a new recipe is followed. There appears to be no predictive function based on real time status of the workpiece. Neither reference provides a predictive model which can avoid future problems regarding the exotherm problem.
Currently available cure techniques are particularly deficient when used with relatively thick parts. These thick parts are particularly susceptible to uncontrollable exotherm, and are presently cured slowly in order to avoid it. A control technique which better managed the curing process for such thick parts would improve product quality while decreasing cure process time for such thick parts.
It would be desirable to provide a process control technique which enables accurate and robust process control for curing fiber composite parts, including relatively thick workpieces. It would also be desirable for such a control technique to provide optimal cure of a workpiece, allowing optimization of Tg, degree of cure, and fiber fraction part parameter values.
In accordance with the present invention, a monitoring and control system for use in curing composite materials includes a model for a workpiece being cured. The model calculates current internal states of the workpiece and predicts, based upon past and current states of the workpiece, future states of the cure process. These future states are represented as virtual inputs to the controller, which controls operation of the cure process based upon both real and virtual inputs. Cure rates are affected by both external temperatures and internal heat generated by the curing process itself. The internally generated heat is considered by the model when calculating current states and predicting future states. By projecting the cure state into the future, problems caused by high cure rates can be avoided. In addition, pressure can be optimally controlled in response to estimated internal material state.